During the physical vapor deposition of wafers, a deposition chamber, as illustrated in FIG. 1, is used to sputter the deposition material on a wafer 4. This deposition material can be any compound which is commonly used in the physical vapor deposition of wafers. Aluminum, an alloy of aluminum, titanium, tungsten and a composite of titanium and tungsten are commonly used as deposition materials. A target 2 is formed out of the deposition material to be used and is positioned at the top of the deposition chamber. The wafer 4 is supported by a pedestal 5 and is positioned at the bottom of the deposition chamber. A shield 6 is positioned within the interior of the deposition chamber. Inside the deposition chamber, the wafer 4 rests on a pedestal 5 which is positioned at the bottom of the deposition chamber.
As is well known, a plasma of a gas, e.g., argon, is formed in the chamber. Ions from the plasma are attracted to the target by applying an appropriate voltage to the target. For example, a plasma can be formed by applying a sufficiently large DC voltage between the cathode, e.g., the target 2, and the anode, e.g., the shield 6. By applying the negative terminal of the voltage supply to the target 2, the ions in the plasma will be attracted to the target 2 as the plasma is formed.
As the plasma ions strike the target 2, particles are sputtered from the surface of the target 2 at a significant kinetic energy. Because of the amount of kinetic energy imparted to the particles escaping from the target 2, the particles will typically adhere securely to any solid structure which they strike, including the interior surface of the shield 6.
A magnetron 1 can be used to shape the plasma and the flow of ions to the target 2. The magnetron 1 can be one or more permanent magnets or electro-magnets of appropriate strength, orientation, and position to achieve the desired shaping. Additionally, the magnetron can be moved during a deposition process, e.g., by a motor to provide uniform plasma flow to the target.
In this deposition chamber it is also possible to conduct reactive sputtering using more than one element to make up the deposition material. To conduct reactive sputtering and deposit a compound consisting of more than one element on the wafer 4, a gas of the second element is introduced into the deposition chamber inside the area enclosed by the shield 6. The first element is still obtained from the target 2 as described above. As the sputtered particles from the target 2 are travelling away from the target 2, they react with the gas particles on their way to the wafer 4 forming a reactive compound which is then deposited on the wafer 4.
Most of the reactive compounds used for reactive sputtering cannot exist in thick layers on an object without breaking up or flaking. For example, compounds such as titanium nitride are commonly used in reactive sputtering, are high stress materials and can only exist in very thin layers without breaking up or flaking. When the layers of these types of reactive compounds become too thick they tend to crack and break apart introducing extraneous and unwanted particles into the deposition chamber.
After the deposition of multiple wafers, the layer of the reactive deposition compound becomes thick on the interior walls of the shield 6. This reactive compound layer will then begin to flake and crack, introducing extraneous particles within the interior of the shield 6. These particles will damage the deposition of future wafers and will detract from the purity of the layer deposited on the surface of the wafer 4.
Because of the particles created from the flaking of thick layers of reactive compounds it is necessary to replace the shield 6 periodically. The deposition chamber cannot be used during the time that the shield 6 is being replaced, costing the owner valuable production throughput time and the cost of the new shield can also be expensive.
A different but related problem results when a collimator 3 is positioned between the target 2 and the wafer 4 inside of the deposition chamber (FIG. 3). The collimator 3 filters the moving particles of the target material so that only particles traveling within a predetermined range of angles can strike the wafer. The collimator 3 has holes 7 which extend through its depth, allowing the particles sputtered from the target 2 to pass through the collimator 3 if they are projected through a hole 7. During deposition the particles sputtered from the target 2 travel through the holes 7 and are deposited on the wafer 4. The collimator 3 also has a surface 8 between each of the holes 7 and the interior sidewalls 9 of the holes. The ratio of the depth of the holes 7 to the diameter of the holes 7 is called the aspect ratio. Some of the particles sputtered from the target 2 will be deposited on top of the surface 8 and on the interior sidewalls 9 of the holes 7 as well as on the interior surface of the shield 6.
After a vacuum is formed, a partial pressure of argon gas is formed in the chamber to aid in the formation of plasma. The partial pressure is typically in the range of 0.5-20 mTorr of the gas, e.g., argon. During a reactive deposition nitrogen gas will also be introduced into the chamber to react with the target material being deposited. As illustrated in FIG. 4, at least a portion of the particles of target material will strike a gas molecule or a plasma ion after such a particle passes through the collimator 3. Occasionally, a particle of target material 12 will ricochet from the gas molecule 14 or ion while losing much of its kinetic energy to the gas molecule as shown in FIG. 4. At least a portion of these slow moving particles will adhere loosely to the underside of the collimator. A layer formed from such slow moving particles is likely to flake more readily than even those on the shield.
What is needed is an apparatus and method which will allow the shield and collimator to be used for a longer period of time in the deposition of reactive compounds without creating flakes and extraneous particles inside of the deposition chamber.